System, apparatus, and method for increasing resiliency in communications

ABSTRACT

A transmitting system inserts runt abort packets in an outgoing data stream during idle time inter-frame time fill. The runt abort packets cause the receiving system to synchronize itself to the transmitting system so that even if an error during inter-frame time fill causes the receiving system to go into an erroneous state, the receiving system will be synchronized with the transmitting system before receiving valid data. In one embodiment, the transmitting system transmits data in packets over SONET. The packet data is scrambled at the transmitting end and descrambled at the receiving end. Runt abort packets sent during inter-frame time fill resynchronize the descrambler. If there is an error in the inter-frame time fill bytes, causing the receiving end descrambler to no longer be synchronized with the transmitting end scrambler, the runt abort packets will cause the descrambler to resynchronize state with the transmitting scrambler.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/752,828 filed Jan. 3, 2001, the entire disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

A. Field of the Invention

Systems, apparatus, and methods consistent with the present inventionrelate generally to data communications and, more particularly, tosystems, apparatus, and methods for increasing resiliency of datacommunications.

B. Description of Related Art

Transmitting data reliably in a network requires transmitting andreceiving systems to process data in the same way. For example, if thetransmitting system scrambles data, the receiving system must descramblethe data in the same manner in which it was scrambled, Standards oftendefine how transmitting and receiving systems should process data. TheSynchronous Optical NETwork (SONET) standard is one such standard.

SONET was developed for transporting data over optical links. In SONET,data is transmitted in a SONET payload envelope (SPE). SONET issometimes used to transport other protocols. For example, standards havebeen developed for transporting Point-to-Point Protocol (PPP) data overSONET. One approach to PPP over SONET is described in a paper entitled“PPP Over SONET (SDH) at Rates From STS-1 (AU-3) TO STS-192c(AU-4-64c/STM-64),” S. Merchant (prepared for the PPP Extensions WorkingGroup, 1998) (“Merchant”), the contents of which are hereby incorporatedby reference.

Merchant describes encapsulating data in a frame format similar to theframe format used in the High-level Data Link Control (HDLC) protocol.The HDLC-like frame is then placed in the SPE for transport over SONET.The data is delineated with a 0x7e flag at the beginning of the data andthe same flag, 0x7e, at the end of the data. Therefore, 0x7e followed bya data value indicates the start of an HDLC frame, and a data valuefollowed by 0x7e indicates the end of an HDLC frame.

A problem arises, however, if a data byte between the 0x7e flags happensto also equal 0x7e. To solve this problem, the data values are analyzed.Each single byte 0x7e data value is replaced, or “escaped,” by thetwo-byte value 0x7d 0x5e. This operation is referred to as byteexpansion because one byte is replaced by two bytes.

On the receiving end, the receiving system passes 0x7e values until anon-0x7e value is received, meaning a data stream has started. When thereceiving system encounters the two-byte 0x7d 0x5e sequence in the data,it converts the sequence back to 0x7e. This solves the problem of flagvalues appearing in the data.

But this raises another problem. How does the receiving end know when0x7d is data, and when it is the first byte of the sequence 0x7d 0x5e?To solve this problem, the transmitting system also replaces each singlebyte 0x7d data value by the two-byte sequence 0x7d 0x5d. The receivingsystem, in addition to converting 0x7d 0x5e in the data back to 0x7e,also converts 0x7d 0x5d back to 0x7d.

Byte expansion solves the problem of flag bytes appearing in the data,but unfortunately raises another. An attacker might use this property tooverload a link by sending a long string of 0x7d and/or 0x7e datavalues. Since these single-byte values are expanded to two bytes, thenumber of bytes is effectively doubled. This reduces bandwidth on thelink by half. Thus, an attacker can reduce the bandwidth by sendingthrough a long string of 0x7d and/or 0x7e data values.

Merchant proposes solving the byte expansion problem by scrambling thedata using an HDLC scrambler. Scrambling the data before the data isanalyzed for flag values virtually eliminates the possibility of anattacker using byte expansion to halve the bandwidth. Thus, scramblingthe values thwarts an attacker sending through all 0x7d's and 0x7e's.

One problem with using an HDLC scrambler, however, is that the receivingend HDLC descrambler might fall out of synchronization with thetransmitting end HDLC scrambler. On the transmitting end, the HDLCscrambler only advances when packet data is coming through. Thus, duringidle time, the HDLC scrambler is not advancing its state and thetransmitting system is sending 0x7e inter-frame time fill bytes in eachSPE.

A problem arises, however, when there is an error in one or more of the0x7e inter-frame time fill bytes. If there is an error in one of theinter-frame time fill bytes, the receiving system interprets the byte asa data value because it no longer has a value of 0x7e. On the receivingend the HDLC descrambler is active only when packet data is beingreceived (e.g., bytes other than 0x7e inter-frame time fill bytes arebeing received). Thus, when there is an error in one of the inter-frametime fill bytes, the byte appears to not be an inter-frame time fill0x7e byte, which causes the HDLC descrambler to advance during idletime, when it should not. This causes the receiving end HDLC descramblerto no longer be synchronized with the transmitting end HDLC scramblerbecause it has advanced based on an error, when it should not advance.

When the next data packet is received at the HDLC descrambler, thepacket will not be descrambled correctly because the HDLC descrambler isnot in the correct state. Consequently, the packet will be thrown out.

Therefore, there exists a need for systems, apparatus, and methods thatmaintain a link between two systems even when there are bit errorsduring slow transmission periods.

SUMMARY OF THE INVENTION

Systems, apparatus, and methods consistent with the present inventionaddress this and other needs by replacing inter-frame time fill byteswith synchronization packets at the transmitting end. When eachsynchronization packet is received at the receiving end, the elements atthe receiving end are resynchronized to the elements at the transmittingend. More particularly, systems, apparatus, and methods consistent withthe present invention insert at the transmitting end runt abort packetsduring inter-frame time fill. The runt abort packets synchronize thedescrambler at the receiving end with the transmitting end scrambler.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate an embodiment of the inventionand, together with the description, explain the invention. In thedrawings,

FIG. 1 is a block diagram of two network elements consistent with theprinciples of the invention;

FIG. 2 is a block diagram of a transmitting system for inserting runtabort packets during inter-frame time fill, consistent with theprinciples of the invention;

FIG. 3 is a flow chart illustrating the process performed by outgoingHDLC processing element 42, consistent with the principles of theinvention;

FIG. 4 is a block diagram illustrating incoming HDLC processing element48, consistent with the principles of the invention;

FIG. 5 is a block diagram showing an embodiment of an HDLC descramblerconsistent with the principles of the invention; and

FIG. 6 is a flow chart illustrating the processing performed by incomingHDLC processing element 46, consistent with the principles of theinvention.

DETAILED DESCRIPTION

The following detailed description of the invention refers to theaccompanying drawings. The same reference numbers in different drawingsidentify the same or similar elements. The following detaileddescription does not limit the invention. Instead, the scope of theinvention is defined by the appended claims and equivalents of the claimlimitations.

Systems, apparatus, and methods consistent with the present inventioninsert information during idle time inter-frame time fill that refreshesa descrambler at the receiving end, thus preventing bit errors duringinter-frame time fill from placing the descrambler in an erroneousstate. In one embodiment, runt abort packets are inserted duringinter-frame time fill and the receiving system flushes a descramblerupon receiving each runt abort packet.

FIG. 1 is a block diagram of two network elements. In this embodiment,network element 36 transmits packets over SONET to network element 38.Network elements 36 and 38 are shown in simplified form for purposes ofillustration and explaining the principles of the invention. In oneembodiment, each of network elements 36 and 38 includes elements fortransmitting and receiving data over a variety of media using a varietyof protocols. In one embodiment, network elements 36 and 38 are routers,each having elements for transmitting data over and receiving data frommany types of media using many types of protocols.

Network element 36 comprises network engine 40, outgoing HDLC processingelement 42, and outgoing SONET processing element 44. Network element 38comprises incoming SONET processing element 46, and incoming HDLCprocessing element 48, and network engine 50. In one embodiment, networkelements 36 and 38 route packets and are connected to other networkelements, networks, and processing systems.

In one embodiment, network elements 36 and 38 transfer data usingpackets over SONET (POS) by framing packet data in an HDLC-like format.One example of POS is described in “PPP in HDLC-like Framing,” RFC-1662,W. Simpson, editor, prepared for the Network Working Group, 1994(“Simpson”), which is hereby expressly incorporated herein by reference.

Outgoing HDLC processing element 42 receives packet data from networkengine 40 and performs HDLC processing on the packet data. Outgoing HDLCprocessing element 42, in addition to performing operations consistentwith principles of the invention, also performs operations similar toconventional HDLC processing elements, such as frame check sequence(FCS) operations and scrambling data.

Consistent with the principles of the present invention, outgoing HDLCprocessing element 42 inserts runt abort packets between inter-frametime fill bytes during inter-frame time fill. The runt abort packets arepreferably short. In one embodiment, the runt abort packets are lessthan six bytes long and may comprise, for example, a series ofcharacters, such as “J”, “N ,” “P,” “R,” “!.”

Each runt abort packet includes an indicator that the receiving endshould abort the packet. In one embodiment, the abort byte sequence atthe end of the packet is 0x7d 0x7e.

After HDLC processing by outgoing HDLC processing element 42, the datais transferred to outgoing SONET processing element 44, which performsSONET processing on the HDLC frames. For example, outgoing SONETprocessing element 44 scrambles the HDLC frames, loads them into a SONETpayload envelope (SPE), and prepares the SONET frame for transport. TheSONET frame is then output to another network element, such as networkelement 38.

Incoming SONET processing element 46 receives the SONET frames, pullsout the HDLC frames from the SPEs, and sends them to incoming HDLCprocessing element 48. Incoming SONET processing element 46 performsconventional SONET processing operations corresponding to the SONETprocessing operations on the transmit side. For example, the SONETpayload is descrambled by incoming processing element 46, and thepayload data is removed from the SPE.

Incoming HDLC processing element 48 receives the HDLC frames and pullsthe packet data out of the frames. Incoming HDLC processing element 48performs many operations corresponding to the HDLC processing operationson the transmit side. For example, the HDLC frame is descrambled byincoming HDLC processing element 48.

When a runt abort packet is received by incoming HDLC processing element48, the runt abort packet is run through a HDLC descrambler of incomingHDLC processing element 48, thus synchronizing the HDLC descrambler tothe HDLC scrambler of outgoing HDLC processing element 42 at thetransmit end. The runt abort packet is discarded because it is tooshort.

During inter-frame time fill, the transmitting end sends out alternatingrunt abort packets and individual 0x7e inter-frame time fill bytes. Inan alternative embodiment, runt abort packets are inserted betweenseveral inter-frame time fill bytes. If there is a bit error in a 0x7einter-frame time fill byte, causing the HDLC descrambler to lose state,the next runt abort packet will flush the HDLC descrambler, placing itback in a correct state, synchronized with the transmit side HDLCscrambler. Thus, even if errors occur in inter-frame time fill bytes,the HDLC descrambler at the receiving end stays synchronized with theHDLC scrambler at the transmitting end.

This solves the problem of conventional systems, which discard the nextpacket after a bit error in the 0x7e inter-frame time fill bytes.Conventional systems only transmit 0x7e bytes during inter-frame timefill. If one of these bytes has an error, it is interpreted as anon-inter-frame time fill byte, causing the HDLC descrambler to advanceduring inter-frame time fill, when it should not advance. The HDLCdescrambler should only advance when there is packet data. Because a biterror causes the descrambler to advance incorrectly during inter-frametime fill, a conventional system will not descramble the next packetcorrectly, and the packet will be discarded even though the packet maybe valid.

FIG. 2 is a block diagram of a transmitting system for inserting runtabort packets during inter-frame time fill, consistent with theprinciples of the invention. More particularly, outgoing HDLC processingelement 42 comprises runt abort insert 20, FCS insert 24, HDLC scrambler28, HDLC abort and flags 30. Data to be transmitted is fed to runt abortinsert 20 from a queue (not shown). If there is data in the queue, runtabort insert 20 forwards the data to FCS insert 24.

During idle time the queue is empty. Since there is no data in thequeue, runt abort insert 20 forwards an alternating sequence ofinter-frame time fill bytes (0x7e) and runt abort packets to FCS insert24. Each runt abort packet is preferably less than six bytes long and ismarked with an indicator that it should be aborted at the receiving end.

In one embodiment, a random number generator is used to generate therunt abort data bytes. This adds randomness to the scrambler state. Ifthere were ever a problem in which packet data was not sufficientlyrandom to ensure a random scrambler state, this would solve thatproblem.

FCS insert 24 receives packet data, runt packets, or inter-frame timefill bytes from runt abort insert 20. If packet data or a runt packetare received, FCS insert 24 runs a frame check sequence over the bytes,appends the FCS result, and forwards the bytes and FCS result to HDLCscrambler 28. Inter-frame time fill bytes are simply forwarded to HDLCscrambler 28.

HDLC scrambler 28 only runs on packet data, not when 0x7e inter-frametime fill bytes are received. Therefore, HDLC scrambler 28 scramblesboth the data that comes from the queue through runt abort insert 20, aswell as the runt abort packets created by runt abort insert 20. In oneembodiment, HDLC scrambler 28 is an x^29+1 self-synchronous scrambler.

HDLC scrambler 28 scrambles the bits and forwards the scrambled bits toHDLC abort and flags 30. HDLC abort and flags 30 adds 0x7e flags to thebeginning and end of the scrambled information to form an HDLC frame. Ifa packet was marked by runt abort insert 20 as a runt abort packet, HDLCabort and flags 30 appends a special two-byte character, 0x7d 0x7e atthe end of the scrambled bytes. This two-byte character indicates to thereceiving end that the packet should be aborted. Finally, HDLC abort andflags 30 forwards the HDLC frames to outgoing SONET processing element44 for SONET processing. SONET processing element 44 performsconventional SON ET processing on the HDLC frames and inter-frame timefill bytes.

FIG. 3 is a flow chart illustrating the process performed by outgoingHDLC processing element 42. Outgoing HDLC processing element insertsrunt abort packets during inter-frame time fill (step 64), calculatesand inserts a frame check sequence based on packet bytes (step 66),scrambles the packet bytes (step 68), and marks runt abort packets andencapsulates the packet bytes with flags to form HDLC frames (step 70).Each of the steps is described in greater detail in the description ofFIG. 2.

FIG. 4 is a block diagram illustrating an embodiment of incoming HDLCprocessing element 48. Incoming HDLC processing element 48 operates ondata received from incoming SONET processing element 46. HDLC processingelement 48 comprises HDLC abort and flag check 80, HDLC descrambler 82,bytes <6 removal 84, and FCS checker 86.

HDLC abort and flag check 80 analyzes the incoming bytes for start offrame, end of frame, and abort sequences. 0x7e values, indicatinginter-frame time fill, are discarded until a non-0x7e value is received,indicating data. Data bytes are forwarded to HDLC descrambler 82.

HDLC abort and flag check 80 checks the incoming bytes to determinewhether a 0x7d 0x7e abort sequence has been added to a packet,indicating the packet should be aborted. All runt abort packets includethis byte sequence. HDLC abort and flag check 80 forwardsnon-inter-frame time fill packet data to HDLC descrambler 82. In oneembodiment, HDLC descrambler 82 is an x^29+1 self-synchronousdescrambler. HDLC descrambler 82 descrambles the data and forwards thedescrambled data to bytes <6 removal 84.

Bytes <6 removal 84 discards packets having a byte length less than sixbytes long. Thus, all runt abort packets are discarded at this point.Packets six bytes or longer are forwarded by bytes <6 removal 84 to FCSchecker 86, which performs FCS operations on the packet and checks theresults against the FCS information appended to the packet.

FIG. 5 is a block diagram showing an embodiment of descrambling elementsof HDLC descrambler 82 consistent with the principles of the invention.More particularly, FIG. 5 shows a particular arrangement of elements fordescrambling bits that may be used in HDLC descrambler 82. HDLCdescrambler 82 may also include other elements for processing theincoming bits.

The descrambling elements comprise 29-bit shift register 120 connectedto exclusive-OR element 122. Scrambled bits from the transmitting endare input to exclusive-OR element 122 and the input of 29-bit shiftregister 120. Exclusive-OR element 122 logically exclusive-ORs theincoming scrambled bits with the output of 29-bit shift register 120,and outputs descrambled data. This operation is the reverse of thescrambling operation performed at the HDLC scrambler at the transmittingend.

29-bit shift register 120 is flushed out by every 29 bits of incomingscrambled data. An error in the bits of scrambled inter-frame time fillbytes causes the shift register to go out of synchronization with thetransmitting end scrambler. The next runt abort packet, however,resynchronizes 29-bit shift register 120 by flushing any errors out ofthe shift register. Even though the runt abort packet will not bedescrambled correctly when there is an error in 29-bit shift register120, the runt abort packet will place the shift register in a correctstate. Subsequent scrambled data will be descrambled correctly. Thisminimizes the impact that errors in inter-frame time fill bytes have ondescrambling data.

FIG. 6 is a flow chart illustrating the processing performed by incomingHDLC processing element 48. Incoming HDLC processing element 48 firstchecks the incoming data for abort and flag information (step 100),descrambles the data (step 102), removes packets having a byte lengthless than six bytes (step 104), and performs FCS checking operations onthe data (step 106). Each of the steps is described in greater detail inthe description of FIG. 4.

CONCLUSION

Systems, apparatus, and methods consistent with the principles of theinvention insert runt abort packets at a transmitting system duringinter-frame time fill to minimize the impact of a bit error duringinterframe time fill. At a receiving system, the runt abort packetsflush a descrambler. Flushing the descrambler removes any false advancesof the descrambler that might have resulted from a bit error in aninter-frame time fill byte.

The foregoing description provides illustration and description, but isnot exhaustive. The invention is not limited to the precise formdisclosed. Modifications and variations are possible in light of theabove teachings or may be acquired from practice of the invention. Thescope of the invention is defined by the claims and equivalents of theclaim limitations.

1. A method for facilitating synchronization in a data transmittingsystem, comprising: receiving data into a queue, where the received datais to be transmitted; determining whether the queue is empty; forwardingdata from the queue when it is determined that the queue is not empty;and forwarding idle time synchronization information when it isdetermined that the queue is empty, where the idle time synchronizationinformation comprises a predetermined number of inter-frame time fillbytes followed by a runt abort packet having less than six bytes.
 2. Themethod of claim 1, where the runt abort packet includes an abort byteindicator at an end of the runt abort packet.
 3. The method of claim 1,where the idle time synchronization information comprises an alternatingsequence of runt abort packets and inter-frame time fill bytes.
 4. Themethod of claim 1, where the runt abort packet is generated using arandom number generator.
 5. The method of claim 2, further comprising:scrambling the runt abort packet and the data; appending the abort byteindicator to the scrambled runt abort packet; and forwarding thescrambled runt abort packet and data to an outgoing processing element.6. A device, comprising: a first outgoing element configured to forwardidle time synchronization information when it is determined that data isnot available for forwarding and further configured to forward the datawhen data is available for forwarding, where the idle timesynchronization information comprises a predetermined number ofinter-frame time fill bytes followed by at least one runt abort packethaving less than a predetermined number of bytes and an abort packetidentifier; and a second outgoing processing element configured toreceive the data and the idle time synchronization information from thefirst outgoing processing element and further configured to generatepacket information by processing the data and the idle timesynchronization information in accordance with a packet protocol.
 7. Thedevice of claim 6, where the second outgoing processing element isfurther configured to scramble the data and the at least one runt abortpacket.
 8. The device of claim 6, where the at least one runt abortpacket is less than six bytes.
 9. The device of claim 6, where the idletime synchronization information comprises an alternating sequence ofinter-frame time fill bytes and runt abort packets.
 10. The device ofclaim 6, where the second outgoing processing element is furtherconfigured to not scramble the at least one inter-frame time fill byte.11. A method, comprising: receiving, via a data receiving system, a datastream from a data transmitting system; identifying idle timesynchronization information in the received data stream, thesynchronization information comprising a predetermined number ofinter-frame time fill bytes followed by a runt abort packet; determiningwhether the idle time synchronization information includes the runtabort packet in the received data stream; discarding the idle timesynchronization information when the idle time synchronizationinformation includes the runt abort packet; and synchronizing the datareceiving system with the data transmitting system by resetting an idlestate of the data receiving system upon receipt of the idle timesynchronization information including the runt abort packet.
 12. Themethod of claim 11, where the identifying idle time synchronizationinformation further comprises: identifying an abort byte sequenceappended to the runt abort packet.
 13. The method of claim 11, where theidle time synchronization information includes an alternating sequenceof inter-frame time fill bytes and runt abort packets.
 14. A system,comprising: an outgoing device configured to insert idle timesynchronization information into a data stream, where the idle timesynchronization information includes a predetermined number ofinter-frame time fill bytes followed by at least a runt abort packet;and a receiving device configured to receive the data stream and toidentify the runt abort packet in the idle time synchronizationinformation, where the receiving device is further configured tosynchronize with the outgoing device by resetting an idle state of thereceiving device upon identification of the runt abort packet.
 15. Thesystem of claim 14, where the idle time synchronization informationcomprises an alternating sequence of inter-frame time fill bytes andrunt abort packets.